Pauly

kenchan is right. ive heard this reffered to as "rolling diameter". usually they will offer you the thinner tire to compensate the growth in wheel size, unless you specify otherwise. and vegas, right again about the power sap on the bigger rim, regardless of the "rolling diameter" remaining the same. why? aside from obvious weight gain? i dont know. but i have experienced this on two vehicles. must, maybe the rims you chose were close enough to the weight of your old rims, so you didnt get power loss? just guessing